Cemented carbides for cutting tools, as composite materials of a WC hard phase and a cobalt (Co) binder metal phase, are representative dispersion alloys, wherein their mechanical properties depend on a particle diameter of the WC hard phase and an amount of the Co binder metal phase, hardness and toughness are particularly in an inversely proportional relationship to each other, properties required for the cemented carbides for cutting tools vary according to machining methods, and accordingly, various attempts have been made to control the mechanical properties of the cemented carbides.
Recently, in machining market, there is a growing demand for a shorter cycle time to improve competitiveness through cost reduction. In order to reduce the cycle time, since machining conditions are gradually changed to high-speed, high-feed conditions, there is an increasing need to allow physical properties of the corresponding cutting tool to have characteristics in which both wear resistance and toughness are good at the same time so that good machining may be performed even under the high-speed, high-feed conditions.
Accordingly, with respect to a hard coating coated on the cutting tool, a coating including an alpha-phase alumina layer, which has excellent stability at high temperature, has been preferred, and, with respect to a MT-TiCN layer formed as an underlayer of the alumina layer, a fine and uniform columnar structure has been preferred due to a trend towards high hardness.
In a case in which non-uniform plastic deformation occurs in a base material of the cutting tool, since chipping easily occurs in a high-hardness film formed on the base material, stability of base material characteristics in a direction perpendicular to the film is required to allow physical properties of the high-hardness film to be fully exhibited.
In a surface portion of the base material on which the hard film is formed, a layer (Cubic phase Free Layer, hereinafter, referred to as “CFL”), in which a cubic carbide constituting the base material is not present, is formed from a surface to a depth of about 10 μm to about 40 μm so as to absorb an impact generated during machining as disclosed in a patent document (Korean Patent Application Laid-open Publication No. 2005-0110822), wherein uniformity of the CFL (uniformity of microstructure by location, uniformity of composition by location) is required for obtaining high hardness of the above-described film.
However, in a CFL of a current commercially available cemented carbide, differences in composition are large, for example, an amount of Co is changed up to about 2 times from a surface of a base material to the inside thereof, and, with respect to hardness of the CFL, differences in hardness between the surface, the CFL, and the inside of the base material are very large according to the large differences in the amount of Co.
Since the significant differences in the composition and hardness according to a thickness of the CFL reduce the stability of the base material, it may be a cause of deteriorating the properties of the high-hardness film formed on the base material. Thus, recently, research and development focusing on reducing the thickness of the CFL are being conducted.
However, since the CFL is basically for absorbing the impact during the machining, the absorption of the impact is reduced to reduce impact resistance of the cutting tool when the thickness of the CFL is significantly reduced. As a result, lifetime of the cutting tool may be reduced.